1. Field of Invention
The present invention relates to a semiconductor process. More particularly, the present invention relates to a method for forming a metal salicide (self-aligned silicide) layer on a shallow junction.
2. Description of Related Art
In a MOSFET (metal-oxide-semiconductor field effect transistor) process, metal suicide, such as tungsten suicide (WSix), is usually formed on gates and sources/drains with a salicide process to reduce the resistance of the devices. The metal silicide formed with a salicide process is called xe2x80x9cmetal salicidexe2x80x9d.
In the prior art, one method for forming tungsten salicide is selective tungsten chemical vapor deposition (W CVD), wherein silane (SiH4) and tungsten hexafluoride (WF6) are used as reactive gases. In a selective W CVD process, tungsten silicide is formed only on the locations where electron transfer occurs, including the substrate and the exposed portion of a gate, but not on insulators like spacers. The tungsten silicide layer in a selective W CVD process is intended to form via the reaction of SiH4 and WF6, however, still a portion of the silicon substrate is consumed by WF6. The consumption of the silicon atoms in the substrate may damage the source/drain junction of the MOSFET device since ultra-shallow junctions are frequently adopted to inhibit short channel effect (SCE) in advanced processes. Consequently, the device suffers from severe current leakage and has relatively low reliability.
Accordingly, this invention provides a method for forming a metal salicide layer on a shallow junction. In the method, few silicon atoms in the substrate are consumed, and the shallow junction therefore is not damaged to cause severe leakage.
The method for forming a metal salicide layer on a shallow junction of this invention comprises the following steps. A substrate having a shallow junction therein is provided. An atomic layer deposition (ALD) process is then performed to deposit a metal salicide layer on the shallow junction. In the ALD process, a gaseous silicon-containing compound and a gaseous metal-containing compound that reacts into metal silicide are introduced alternatively onto the substrate, wherein either compound can be introduced at first.
If the silicon-containing compound is introduced at first, the molecules are adsorbed on the substrate over the shallow junction, and the flow rate of the silicon-containing compound is controlled so that only a single layer of molecules is adsorbed. Then, a pulse of the metal-containing compound is introduced to react with the adsorbed silicon-containing compound to form a thin layer of metal silicide, wherein the flow rate of the metal-containing compound is controlled so that few silicon atoms in the substrate are consumed. By repeating the two gas introduction steps, a metal salicide layer constituted of many thin layers is formed on the shallow junction.
If the metal-containing compound is introduced at first, the molecules are adsorbed on the substrate over the shallow junction, and the flow rate of the metal-containing compound is controlled so that only a single layer of molecules are adsorbed and few silicon atoms in the substrate are consumed. A pulse of the silicon-containing compound is introduced to react with the adsorbed metal-containing compound to form a thin layer of metal silicide. By repeating the two gas introduction steps, a metal salicide layer constituted of many thin layers is formed on the shallow junction.
The substrate may further comprise a gate structure thereon comprising a gate dielectric layer, a polysilicon gate and a spacer, and the metal salicide layer is also formed on the polysilicon gate. Moreover, in the aforementioned method for forming a metal salicide layer on a shallow junction, argon may be introduced after each gas introduction step to remove excess reactive gas and improve the quality of the metal salicide layer.
Since few silicon atoms in the substrate are consumed in this invention with a controlled flow rate of the metal-containing compound, the shallow junction is not damaged, and current leakage that would lower the reliability of the device does not occur.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.